The present invention relates to a method for the preparation of microcapsules having excellent properties or, more particularly, to a novel method for the preparation of dense microcapsules having excellent heat resistance, water resistance and solvent resistance by the method of so-called in situ formation.
As is known, microcapsules are widely used as a coating material of pressure-sensitive transfer paper and as a mounting medium of medicines, agricultural chemicals, perfumes, adhesives, activated charcoals, enzymes, dyes, solvents and the like.
Hitherto known methods for the preparation of microcapsules include so-called coacervation method, interfacial polymerization method, in situ method and others, of which the in situ method is promising in most cases in respect of the low manufacturing costs in a relatively simple procedure in addition to the versatility relative to the type of the core material to be encapsulated.
Microcapsule products are required in recent years to have upgraded performance in the resistance against high temperature, high humidity and solvents as well as extended durability. These requirements are partly satisfied by the in situ method in which the capsule walls are formed from an amino resin. For example, a proposal has been made in Japanese Patent Kokai No.53-84881 for the method in which the capsule walls of a melamine-formaldehyde resin is formed in the presence of a copolymer of an ethylenically unsaturated monomer and maleic anhydride. Further, Japanese Patent Publication No. 60-2100 discloses a method for forming capsule walls of a melamine-formaldehyde resin in an acidic aqueous solution of a copolymer of styrene and maleic anhydride.
Though excellent in the resistance against heat, water and solvents, the capsule walls of a melamine-formaldehyde resin formed in these prior art methods sometimes cause troubles due to their poor denseness that leakage of the core material inside takes place out of the microcapsules and the core material is denaturated by water or solvent permeating into the microcapsules through the capsule walls.
An alternative method is proposed in Japanese Patent Kokai No. 51-144383 according to which an oleaginous material is coated with a coating material prepared in advance from a condensation product of formaldehyde and a hydroxy-containing polymer or a copolymer of an ethylenically unsaturated monomer and maleic anhydride. This method is also not quite effective when capsule walls having high denseness are desired.